Multilayer structure for reducing film roughness in magnetic devices

ABSTRACT

A seed layer stack with a smooth top surface having a peak to peak roughness of 0.5 nm is formed by sputter depositing an amorphous layer on a seed layer such as Mg where the seed layer has a resputtering rate 2 to 30× that of the amorphous layer. The uppermost seed layer is a template layer that is NiCr or NiFeCr. As a result, perpendicular magnetic anisotropy in an overlying magnetic layer that is a reference layer, free layer, or dipole layer is substantially maintained during high temperature processing up to 400° C. and is advantageous for magnetic tunnel junctions in embedded MRAMs, spintronic devices, or in read head sensors. The amorphous seed layer is SiN, TaN, or CoFeM where M is B or another element with a content that makes CoFeM amorphous as deposited. The seed layer stack may include a bottommost Ta or TaN buffer layer.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. No. 8,541,855and U.S. Pat. No. 8,871,365; both assigned to a common assignee andherein incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to magnetic devices that utilize thinfilms with perpendicular magnetic anisotropy (PMA), and in particular,to the use of a multilayer stack for the seed layer to increase thermalstability in the resulting magnetic tunnel junction (MTJ) found inembedded MRAM devices that are subjected to high temperaturesemiconductor processes up to about 400° C.

BACKGROUND

Magnetoresistive Random Access Memory (MRAM) has a read function basedon a tunneling magnetoresistive (TMR) effect in a MTJ stack wherein atunnel barrier is formed between a free layer and a reference layer. Thefree layer serves as a sensing layer by switching the direction of itsmagnetic moment in response to external fields (media field) while thereference layer has a fixed magnetic moment. The electrical resistancethrough the tunnel barrier (insulator layer) varies with the relativeorientation of the free layer moment compared with the reference layermoment and thereby provides an electrical signal that is representativeof the magnetic state in the free layer. In MRAM, the MTJ is formedbetween a top conductor (electrode) and bottom conductor. When a currentis passed through the MTJ, a lower resistance is detected when themagnetization directions of the free and reference layers are parallel(“0” memory state), and a higher resistance is noted when they areanti-parallel (“1” memory state). The TMR ratio is dR/R where R is theminimum resistance of the MTJ, and dR is the difference between thelower and higher resistance values. The tunnel barrier is typicallyabout 10 Angstroms thick so that a current through the tunnel barriercan be established by a quantum mechanical tunneling of conductionelectrons.

Another version of MRAM that relies on a TMR effect, and is referred toas a spintronic device that involves spin polarized current, is calledspin-transfer torque (STT) MRAM and is described by C. Slonczewski in“Current driven excitation of magnetic multilayers”, J. Magn. Magn.Mater. V 159, L1-L7 (1996). J-G. Zhu et al. has described anotherspintronic device called a spin transfer oscillator (STO) in “MicrowaveAssisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1,pp. 125-131 (2008) where a spin transfer momentum effect is relied uponto enable recording at a head field significantly below the mediumcoercivity in a perpendicular recording geometry.

MTJ elements wherein one or both of the free layer and reference layerhave perpendicular magnetic anisotropy (PMA) are preferred over theircounterparts that employ in-plane anisotropy because the former has anadvantage in a lower writing current for the same thermal stability, andbetter scalability for higher packing density which is one of the keychallenges for future MRAM applications. In MTJs with PMA, the freelayer has two preferred magnetization orientations that areperpendicular to the physical plane of the layer. Without externalinfluence, the magnetization or magnetic moment of the free layer willalign to one of the preferred two directions, representing information“1” or “0” in the binary system. For memory applications, the free layermagnetization direction is expected to be maintained during a readoperation and idle, but change to the opposite direction during a writeoperation if the new information to store differs from its currentmemory state. The ability to maintain free layer magnetization directionduring an idle period is called data retention or thermal stability. Thelevel of stability required is usually related to the memoryapplication. A typical non-volatile memory device may require thermalstability at 125° C. for about 10 years.

Moreover, for MRAM devices that are often embedded in ComplementarySilicon Oxide Semiconductor (CMOS) chips, the MTJ must be able towithstand high temperature processing conditions up to about 400° C.that are commonly applied during the deposition of low-k dielectricfilms for transistors in CMOS structures. In most cases, thistemperature exceeds the optimum temperature for best magneticperformance in the MTJ or MRAM. MTJs are usually annealed in the300-330° C. degree range to obtain the desired magnetic properties.

As a result of 400° C. processing, free layer PMA is typically reducedand thermal stability is less compared with a condition where the MTJ isannealed only to 330° C., for example. Free layer coercivity is alsoless after high temperature processing to around 400° C. than after300-330° C. annealing. However, it is an important requirement tomaintain coercivity after high temperature processing.

Thus, there is a significant challenge to maintain PMA and enhancethermal stability of reference and free layers to improve theperformance of MTJs at elevated temperatures typical of back end of line(BEOL) semiconductor processes. Current MTJ structures fail to satisfythe performance requirements for advanced embedded MRAM devices.Therefore, an improved MTJ stack is needed to enable a magnetic layerwith thermal stability to at least 400° C., and where PMA is maintainedin the reference layer and free layer.

SUMMARY

One objective of the present disclosure is to provide a multilayer MTJstack in a magnetic device wherein PMA in the magnetic layer adjoiningthe seed layer is maintained or enhanced after high temperatureprocessing of about 400° C. for at least 30 minutes.

A second objective of the present disclosure is to provide a method offorming the MTJ stack that satisfies the first objective.

According to one embodiment of the present disclosure, these objectivesare achieved by configuring a MTJ stack with a seed layer, referencelayer (RL), tunnel barrier, and free layer (FL) in a seedlayer/RL/tunnel barrier/FL bottom spin valve configuration. A keyfeature is the multilayer stack that is selected for the seed layer. Inone embodiment, the seed layer is a stack of four layers wherein abottommost layer such as Ta or TaN is employed for good adhesion to asubstrate or a bottom electrode. A second seed layer contacts a topsurface of the bottommost layer and is selected because of a highresputtering rate property. The second layer is preferably one of Mg,Al, Si, C, B, Mn, Rb, Zn, and Ti and typically has a substantiallyuneven top surface after deposition. Next, a third seed layer that is anamorphous material with a lower resputtering rate than the second layeris formed on the second layer. During the third layer deposition, aportion of the second layer top surface is removed due to a highresputter rate and is replaced by a third layer with less roughness(better peak to peak uniformity) in its top surface. As a result, eachof the second and third seed layers has a smooth top surface withreduced roughness and the combination thereof is called a “smoothinglayer”. The stack of second and third seed layers may be repeated. Theuppermost layer in the seed layer stack serves as a template layer forthe overlying PMA layer. In other words, the uppermost layer is made ofa material such as NiCr or NiFeCr having a (111) crystal orientationthat promotes PMA in the overlying magnetic layer which may be areference layer in a bottom spin valve structure or a free layer in aMTJ with a top spin valve design. Because of a smooth top surface on theamorphous third seed layer, the template layer also has a smooth topsurface with peak to peak roughness ≦0.5 nm over a range of 100 nmcompared with a peak to peak roughness of about 2 nm over a range of 100nm in prior art seed layer films as determined by transmission electronmicroscope (TEM) measurements.

In a bottom spin valve embodiment, the reference layer also known as apinned layer in a synthetic antiparallel (SyAP) stack adjoins a topsurface of the template layer and preferably has intrinsic PMA derivedfrom a laminated stack represented by (Co/X)_(n) where X is Pt, Pd, Ni,NiCo, Ni/Pt, or NiFe, and n is from 2 to 30. In another aspect, CoFe orCoFeR may replace Co in the laminated stack where R is one of Mo, Mg,Ta, W, or Cr. The smooth template layer formed on the smooth top surfaceof the amorphous layer is advantageously used to maintain or enhance PMAin the reference layer after high temperature processing up to about400° C.

A tunnel barrier is formed on the reference layer. In an alternativeembodiment, a transition layer such as CoFe/Co or CoFeB/Co is insertedbetween the reference layer and tunnel barrier. The tunnel barrier ispreferably an oxide, nitride, or oxynitride of one or more of Mg, MgZn,Ta, Ti, Zn, Al, or AlTi.

A free layer/capping layer stack is formed on the tunnel barrier. Thefree layer may be selected from one of the laminated compositionsdescribed with respect to the reference layer. In an alternativeembodiment, the free layer may be one or more of Co, Fe, CoFe, andalloys thereof with one or both of Ni and B. In another aspect, a momentdiluting layer (L) such as Ta or Mg is inserted in one of theaforementioned metals or alloys to give a CoFeB/L/CoFeB configuration,for example. The capping layer may comprise a metal oxide such as MgO toenhance PMA in the free layer by generating perpendicular interfacialanisotropy at a free layer/metal oxide interface. Moreover, there may bean uppermost layer that is one or more of Ru and Ta to give a cappinglayer stack that is MgO/Ru/Ta or the like.

In a second embodiment, the MTJ layers and compositions thereof areretained from the first embodiment but are formed in a top spin valvedesign represented by a seed layer/free layer/tunnel barrier/referencelayer/capping layer configuration. Here, the uppermost template layer inthe seed layer stack adjoins a bottom surface of the free layer.

A third embodiment retains the bottom spin valve stack from the firstembodiment and further includes a spacer/underlayer/PMA layer stackbetween the free layer and capping layer where the PMA layer serves as adipole layer to reduce the offset of the minor switching loop of thefree layer caused by a dipole field from the reference layer. The spacermay be Ta, and the PMA layer is preferably a multilayer stack such as(Co/X)_(n) described previously. The underlayer is the multilayer seedstack described earlier in order to maintain PMA in the dipole layerfollowing high temperature processing.

In the aforementioned embodiments, the buffer layer in the seed layerstack is optional. Thus, the present disclosure anticipates that thesecond seed layer having a high resputter rate may contact a top surfaceof the substrate or the bottom electrode. Moreover, the second and thirdseed layers may be repeated on the substrate to give a laminatedstructure before the uppermost (template) seed layer is deposited.

After all layers in the MTJ are laid down, an anneal process up to 400°C. for 30 minutes may be employed to further improve PMA properties andthereby increase Hc and Hk in the magnetic layers. Thereafter, aconventional process sequence is performed to fabricate a top electrodeon the MTJ stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a seed layer stack in the prior artwhere a rough top surface in the bottom buffer layer is reproduced inthe upper template layer.

FIG. 2 is a cross-sectional view of the seed layer stack with fourlayers formed according to an embodiment of the present disclosure.

FIG. 3a is a cross-sectional view of the seed layer stack in FIG. 2wherein the bottommost buffer layer is omitted according to a secondembodiment of the present disclosure.

FIG. 3b is a cross-sectional view of a seed layer stack wherein the highresputtering rate and low resputtering rate (amorphous) layers arerepeated according to another embodiment of the present disclosure.

FIG. 4 is a cross-sectional view of a magnetic tunnel junction (MTJ)with a bottom spin valve configuration, and containing a seed layerstack formed according to an embodiment of the present disclosure.

FIG. 5 is a cross-sectional view of a MTJ with a top spin valveconfiguration and a seed layer stack formed according to an embodimentof the present disclosure.

FIG. 6 is a cross-sectional view of a MTJ with a bottom spin valveconfiguration and including a dipole layer according to an embodiment ofthe present disclosure.

FIG. 7 depicts a process of sputter depositing an amorphous seed layeron a seed layer with a higher resputtering rate according to anembodiment of the present disclosure.

FIG. 8 is a plot of Kerr signal vs. PMA field for a conventional seedlayer/reference layer stack, and for a seed layer stack/reference layerof the present disclosure after a 400° C., 30 minute anneal.

FIG. 9 is a plot of Kerr signal vs PMA field for the samples in FIG. 7after an additional anneal of 390° C. for 300 minutes.

DETAILED DESCRIPTION

The present disclosure is a MTJ wherein at least one of a free layer,reference layer, or dipole layer has perpendicular magnetic anisotropythat is maintained during 400° C. processing in magnetic devices such asembedded MRAM and STT-MRAM, in spintronic devices such as microwaveassisted magnetic recording (MAMR) and spin torque oscillators (STO),and in various spin valve designs including those found in read headsensors. PMA is maintained by depositing the magnetic layer on a seedlayer stack wherein an uppermost template layer has an unusually smoothtop surface made possible by deposition of the template layer on asmoothing layer comprised of a lower seed layer with high resputteringrate and an upper amorphous layer with a low resputtering rate asdescribed herein. The seed layer stack may be used in devices based onbottom spin valve, top spin valve, or dual spin valve designs asappreciated by those skilled in the art.

Referring to FIG. 1, a cross-sectional view is shown of a seed layerstack formerly fabricated by the inventors. The seed layer is formed ona substrate such as a bottom electrode 10 in a MRAM device, and has abottom layer called a buffer layer 20 that is used for good adhesion tothe substrate. Ta or TaN are commonly selected for the buffer layer,which tends to have considerable roughness at its top surface 20 t. Anupper template layer 23 made of NiCr, NiFeCr, or the like is conformallydeposited on the buffer layer, and has a (111) crystal structure inorder to promote a fcc (111) crystal orientation in an overlying(Co/X)_(n) multilayer. As a result, the significant roughness in topsurface 20 t is essentially duplicated in the top surface of thetemplate layer where peaks 23 s 1 are separated by valleys 23 s 2 in thefilm. The substantial peak to peak roughness in the template layer topsurface is associated with a loss in PMA in an overlying magnetic layer(not shown) such as a reference layer, free layer, or dipole layerduring high temperature processing. Peak to peak roughness is defined asthe average difference in the z-axis direction between peaks 23 s 1 andis about 2 nm over a range r of 100 nm.

Previously, we described in related U.S. Pat. No. 8,871,365 how thermalstability in a reference layer and free layer may be improved with aRL1/DL1/Ru/DL2/RL2 or FL1/DL1/Ru/DL2/FL2 configuration, respectively,where R1 and R2 (or FL1 and FL2) are two magnetic layers with PMA thatare antiferromagnetically coupled through a middle Ru layer. Dustinglayers (DL1 and DL2) such as Co or CoFe are responsible for enhancingthermal stability compared with a RL or FL having a R1/Ru/R2, orFL1/Ru/FL2 stack, respectively.

We also disclosed in related U.S. Pat. No. 8,541,855 how a Hf/NiCr orHf/NiFeCr seed layer improves PMA in an overlying (Co/Ni)_(n)multilayer. Now we have discovered that PMA in a (Co/Ni)_(n) laminate orthe like may be maintained during high temperature processing to about400° C. by a stack of seed layers which promote a more uniform topsurface on the uppermost template layer. In this context, the term“about 400° C.” means the temperature may exceed 400° C. by 10-20° C.for a certain period of time due to temperature fluctuations orexcursions in the chamber where an annealing or deposition process isperformed.

According to a first embodiment depicted in FIG. 2, the seed layer stack24 of the present disclosure includes a bottom layer 20 and an uppermosttemplate layer 23 as previously described. However, a key feature is aso-called “smoothing layer” structure having a stack of layers 21/22where second layer 21 is made of a material with a high resputteringrate that is formed on a top surface 20 t of the bottommost layer. Layer21 preferably is one or more of Mg, Al, Si, C, B, Mn, Rb, Zn, and Tiwith a thickness from 3 to 100 Angstroms, and preferably 3 to 20Angstroms. Non-crystalline or nano-crystalline (grain size <5 nm) layer22 is made of TaN, SiN, or CoFeM where M is one of B, P, Ta, Zr, Si, Cu,Hf, Mo, W, and Nb with an M content that results in amorphous characterfor the CoFeM alloy. Preferably, the CoFeM alloy is amorphous asdeposited. Layer 22 has a thickness from 1 to 100 Angstroms, andpreferably 2 to 15 Angstroms, and has a lower resputtering rate thansecond layer 21 such that layer 21 has a resputtering rate from 2 to 30times that of layer 22.

As defined herein, resputtering rate is related in part to bond energy,which is the energy needed to break apart bonded atoms. Therefore, amaterial with a low bond energy is easy to resputter and has a higherresputtering rate than a material with a higher bond energy. Forexample, the bond energy of Mg—Mg is 11.3 kJ/mol while the bond energyof Fe—Fe is 118 kJ/mol and of Co—Co is about 127 kJ/mol according to atable of values found in “Comprehensive Handbook of Chemical BondEnergies”, Y. Luo, CRC Press, Boca Raton, Fla., 2007. It follows thatthe bond energy ratio between Mg and CoFe (or CoFeB) is about 1:10 togive a resputtering rate for Mg that is about 10 times greater than thatfor CoFe. Thus, the material in layer 21 has a first bond energy that isless than a second bond energy for the material in layer 22.

A second important factor in determining resputtering rate is the atomicnumber (Z) of an element. In particular, materials in layer 21 are moreeasily displaced during deposition of layer 22 when the material for thenon-crystalline or nano-crystalline layer has a higher weight (larger Zvalue) than the element or alloy selected for layer 21. Accordingly, agreater resputtering rate ratio (layer 21/layer 22) is achieved with acondition where layer 21 is an element or alloy with both of a lower Zvalue and smaller bond energy than the material in layer 22.

As a result of the resputtering rate (bond energy) difference, when thenon-crystalline or nano-crystalline material is deposited as depicted inthe deposition sequence found in FIG. 7, atoms of second layer 21 aredisplaced from a top surface of the second seed layer and are replacedby a more uniform film of layer 22. In other words, an “as deposited”top surface 21 r of the second layer with a peak to peak roughness v1becomes a smooth top surface 21 t with substantially less roughness aslayer 22 is deposited thereon. Peak to peak roughness v2 between peaks22 t has been observed to be only 0.5 nm over a 100 nm range r, and issubstantially less than a peak to peak roughness v1 of about 2 nm over a100 nm range for top surface 21 r prior to deposition of layer 22.

Returning to FIG. 2, top surfaces 21 t, 22 t of layers 21 and 22,respectively are shown with a relatively smooth profile compared withthe uneven (rough) top surface 20 t of the bottom seed layer 20.Furthermore, the smooth top surface 22 t is essentially reproduced intop surface 23 t of the uppermost template layer 23 that typicallyconforms to the top surface of the underlying layer. As statedpreviously, the term “smooth” when referring to a top surface 23 t maybe described in terms of a peak to peak roughness over a range of 100nm. In this case, a TEM measurement indicates a peak to peak roughnessin top surface 23 t of about 0.5 nm, which represents a substantialimprovement over the 2 nm value for peak to peak roughness for peaks 23s 1 of the template layer in the FIG. 1 reference.

In a second embodiment illustrated in FIG. 3a , bottom layer 20 may beomitted to provide a seed layer stack 25-1 where the high resputteringrate layer 21 contacts a top surface of the substrate 10. As a result ofdepositing the non-crystalline or nano-crystalline layer 22, both seedlayers in the smoothing layer stack have smooth top surfaces 21 t, 22 tsimilar to that found for the template layer as described previously forthe first embodiment. Thus, the seed layer stack 25-1 is a trilayer witha 21/22/23 configuration where template layer 23 has a top surface 23 tand a peak to peak roughness v2 that is attributed to maintaining PMA inan overlying magnetic layer after processing at temperatures as high as400° C. as supported by data provided in a later section.

The present disclosure also anticipates the smoothing layer stack 21/22may be repeated to give a laminate consisting of alternating layers 21and 22. A third embodiment is depicted in FIG. 3b where a first highresputtering rate layer 21 a is formed on the substrate 10. Above layer21 a is formed sequentially a first low resputtering rate layer 22 a, asecond high resputtering rate layer 21 b, and a second low resputteringrate layer 22 b, and a template layer 23 to give a 21 a/22 a/21 b/22b/23 configuration for seed layer stack 25-2. The bond energy for thematerial in layers 21 a, 21 b is less than that of the material inlayers 22 a, 22 b. In some embodiments, layers 21 a, 21 b may be made ofthe same element or alloy, and layers 22 a, 22 b are selected from thesame material. However, the present disclosure anticipates that layer 21a may have a different composition than layer 21 b, and layer 22 a mayhave a different composition than layer 22 b.

It is believed that the third embodiment provides a further improvementin top surface 23 t uniformity compared with the previous embodiments.In this seed layer design, each high resputtering rate layer preferablyhas a thickness from 3 to 20 Angstroms, and each low resputtering ratelayer 22 a, 22 b with amorphous character preferably has a thickness of2 to 15 Angstroms. It should be understood that the seed layer structurein the first embodiment may be modified accordingly to insert theaforementioned laminated smoothing layer instead of one of each layer21, 22 between layers 20 and 23 in stack 24. Moreover, there may be morethan one repeat of layers 21, 22 in a smoothing layer stack.

The present disclosure also encompasses a magnetic tunnel junction (MTJ)element comprising a seed layer stack formed according to one of theembodiments described herein. In the exemplary embodiments, a bottomspin valve and top spin valve are depicted. However, the seed layerembodiments described herein may be implemented in other spin valvedesigns including a dual spin valve structure as appreciated by thoseskilled in the art.

Referring to FIG. 4, MTJ 1 is formed between a substrate 10 that may bea bottom electrode, and a top electrode 30. A bottom spin valveconfiguration is shown wherein the seed layer stack 24, a referencelayer 26, tunnel barrier 27, free layer 28, and capping layer 29 aresequentially formed on the substrate. In one preferred embodiment, thereference layer (RL) has a synthetic antiparallel (SyAP) stack with anAP2 layer 26 a contacting a top surface of the seed layer, a middlecoupling layer 26 b such as Ru, and an uppermost AP1 layer 26 c.Preferably, both of the AP2 and AP1 layers have PMA such thatmagnetization 26 m 1, 26 m 2, respectively, are aligned in a directionperpendicular to the planes of the MTJ layers. PMA in each of the AP2and AP1 layers may be intrinsic and derived from a laminated stack(Co/X)_(n) where X is Pt, Pd, Ni, NiCo, Ni/Pt, or NiFe, and n is from 2to 30. In another aspect, CoFe or CoFeR may replace Co in the laminatedstack and R is one of Mo, Mg, Ta, W, or Cr. The smooth template layerformed on the top surface of the non-crystalline or nano-crystallinelayer is advantageously used to maintain PMA in the reference layerafter high temperature processing up to about 400° C. In alternativeembodiments, seed layer stack 25-1 or 25-2 is substituted for stack 24.

In other embodiments, the reference layer 26 may have a SyAPconfiguration represented by RL1/DL1/Ru/DL2/RL2 as disclosed in relatedU.S. Pat. No. 8,871,365. In the present disclosure, R1 corresponds tothe AP2 layer and R2 is the AP1 layer described above that areantiferromagnetically coupled through the Ru layer.

There may be a transition layer (not shown) such as CoFe/Co or CoFeB/Coformed between the uppermost laminated layer in a (Co/X)_(n) stack andthe tunnel barrier 27. According to one embodiment, the transition layeris formed between the (111) AP1 layer and a (100) MgO tunnel barrier,and is sufficiently thin to preserve the PMA property of the AP1 layerand yet thick enough to provide a high magnetoresistance (MR ratio). Cois preferably used as the uppermost layer in a transition layer andforms an interface with the tunnel barrier layer since it is moreresistant to oxidation than a CoFeB or CoFe layer. The transition layer,when present, is considered part of the reference layer 26 because ofthe magnetic character in the CoFe/Co and CoFeB/Co layers.

A tunnel barrier 27 is formed on the reference layer 26. The tunnelbarrier is preferably an oxide, nitride, or oxynitride of one or more ofMg, MgZn, Ta, Ti, Zn, Al, or AlTi. The thickness and extent of oxidationin the metal oxide layer may be adjusted to tune the resistance×area(RA) value for the tunnel barrier. It is believed that the smoothness ofthe template layer top surface 23 t is substantially duplicated in theoverlying layers in MTJ 1 including the tunnel barrier.

A free layer/capping layer stack is formed on the tunnel barrier. Thefree layer 28 may be selected from one of the laminated compositionsdescribed with respect to the reference layer. In an alternativeembodiment, the free layer may be one or more of Co, Fe, CoFe, andalloys thereof with one or both of Ni and B. In another aspect, a momentdiluting layer (L) such as Ta or Mg is inserted in one of theaforementioned metals or alloys to give a CoFeB/L/CoFeB configuration,for example. Furthermore, the free layer (FL) may have aFL1/DL1/Ru/DL2/FL2 configuration where FL1 and FL2 are two magneticlayers or a laminate with PMA as previously described that areantiferromagnetically coupled through a middle Ru layer. DL1 and DL2 aredusting layers as explained earlier.

In some embodiments, the capping layer 29 is a metal oxide such as MgOor MgTaOx to enhance PMA in the free layer by inducing interfacialperpendicular anisotropy along an interface with the free layer. Inother embodiments, the capping layer has an uppermost layer that is oneor more of Ru and Ta to give a capping layer stack that is Ru/Ta/Ru orMgO/Ru/Ta, for example.

Referring to FIG. 5, a top spin valve embodiment shown as MTJ 2 isformed according to the present disclosure. Seed layer 24 (or 25-1 or25-2) is formed on the substrate 10 and then free layer 28, tunnelbarrier 27, reference layer 26, and capping layer 29 are sequentiallylaid down on the free layer. When the reference layer has a SyAPconfiguration, the AP1 layer 26 c contacts the tunnel barrier and AP2layer 26 a is the uppermost layer in the reference layer stack. The freelayer contacts the top surface 23 t of the template layer in the seedlayer stack and has a smooth top surface wherein the peak to peakthickness variation value associated with top surface 23 t is believedto be substantially reproduced in the top surface 28 t of the freelayer. The free layer may comprise two magnetic layers FL1 28 a and FL228 c that are antiferromagnetically coupled through layer 28 b that ispreferably Ru. As a result, magnetization 28 m 1 and 28 m 2 in the FL1and FL2 layers, respectively, are perpendicular to the plane of thelayers and aligned in opposite directions. Each of FL1 and FL2 may be a(Co/X)_(n) laminate as described earlier with respect to the referencelayer, or one or both of FL1, FL2 may be one or more of Co, Fe, CoFe,and alloys thereof with one or both of Ni and B. Furthermore, the freelayer may have a FL1/DL1/Ru/DL2/FL2 configuration.

In another bottom spin valve embodiment illustrated in FIG. 6, MTJ 1 ismodified to give MTJ 3 by inserting a second seed layer stack 25-1 and aPMA layer that serves as a dipole layer 32 between the free layer 28 andcapping layer 29. Thus, the MTJ has a first seed layer (SL1) stack 24(or 25-1 or 25-2) contacting a top surface of the substrate 10, and thesecond seed layer (SL2) stack contacting a top surface of a spacer 31 ina SL1/RL/tunnel barrier/FL/spacer/SL2/dipole layer/capping layerconfiguration. The spacer is a material including but not limited to oneof Ta and Mg that getters oxygen from the free layer. The second seedlayer stack that optionally is layer 24 or 25-2 is employed as anunderlayer for the PMA layer to maintain the PMA therein after hightemperature processing. The dipole layer is preferably a (Co/X)_(n)laminate with a composition that is one of the multilayers previouslydescribed with respect to layer 26 in MTJ 1.

The present disclosure also encompasses a method of forming the seedlayer stack in the embodiments disclosed herein. All layers in the MTJstack including the seed layers may be deposited in a DC sputteringchamber of a sputtering system such as an Anelva C-7100 sputterdeposition system or the like that includes ultra high vacuum DCmagnetron sputter chambers with multiple targets and at least oneoxidation chamber. Typically, the sputter deposition process for theseed layer stack including the high resputtering rate layer 21 and lowresputtering rate layer 22 involves an inert gas such as Ar and a basepressure between 5×10⁻⁸ and 5×10⁻⁹ torr. A lower pressure enables moreuniform films to be deposited. The temperature in the sputter depositionchamber during deposition processes may vary from 100° K to 400° C., andthe forward power applied to one or more targets to form each seed layeris usually in the range of 20 W to 5000 W.

The tunnel barrier and metal oxide (when included) for the capping layerare prepared by first depositing a first metal layer, oxidizing thefirst metal layer with a natural oxidation (NOX) or radical oxidation(ROX) process, and then depositing a second metal layer on the oxidizedfirst metal layer. During a subsequent annealing step, oxygen migratesinto the second metal layer to oxidize the second metal. In someembodiments, one or more additional metal layers are deposited in thetunnel barrier stack and each oxidized by a NOX or ROX process before anuppermost metal layer is deposited and then oxidized by way of annealingto generate tunnel barrier 27.

Once all of the layers in the MTJ are formed, an annealing process isperformed that is comprised of a temperature between 330° C. and 400° C.for a period of 1 minute to 10 hours. Thereafter, the spin valve stackmay be patterned to form a plurality of MTJ elements on the substrate 10by a well known photolithography and etch sequence. In an embodimentwhere the substrate is a bottom electrode, the bottom electrode in somecases is patterned simultaneously with the overlying spin valve stack toenable a higher density of patterned structures for advanced technologydesigns.

Example 1

To demonstrate the advantages of the present disclosure, a (Co/Ni),multilayer stack with PMA where n=3 was fabricated on two different seedlayers. The seed layer in the reference sample, which represents theFIG. 1 structure, has a TaN20/NiCr50 stack formed on a first wafer wherethe TaN thickness is 20 Angstroms and the NiCr thickness is 50Angstroms. A second seed layer taken from the FIG. 2 embodiment has aTaN20/Mg7/CoFeB10/NiCr50 stack formed on a second wafer where Mg (7Angstroms thick) is the high resputtering rate layer 21 and CoFeB (10Angstroms thick) is the amorphous layer 22. Each wafer was annealed at400° C. for 30 minutes and a Kerr microscope was used to measure a Kerrsignal vs perpendicular field as illustrated in FIG. 8 where curve 50 isthe signal from the reference wafer, and curve 51 is obtained from thewafer with the seed layer stack formed according to the first embodimentdepicted in FIG. 2. The curves show PMA intensities that are essentiallythe same.

Thereafter, the wafers were annealed at 390° C. for 300 minutes and asecond plot of Kerr signal vs. perpendicular field was obtained as shownin FIG. 9. There is only a slight degradation in PMA compared with thefirst Kerr measurement for the FIG. 2 embodiment according to curve 51a. However, the reference sample exhibits significant PMA degradation asa result of the second anneal step as indicated by curve 50 a. Thus, theseed layer stack of the present disclosure is beneficial insubstantially maintaining PMA in an overlying magnetic layer duringprolonged heating at about 400° C. while the reference sample fails tomaintain a substantial PMA during the same annealing period.

Example 2

In a second experiment that demonstrates the benefit of reduced peak topeak roughness in a template layer top surface provided by a seed layerstack of the present disclosure, a seed layer stack with aTaN20/Mg7/NiCr50 configuration previously fabricated by the inventors,and where the number following each layer is the thickness in Angstroms,was formed on a substrate. For comparison, a laminated smoothing layerhaving a 21 a/22 a/21 b/22 b stack according to the third embodiment wasdeposited and the TaN/Mg/NiCr seed layer stack deposited thereon to givea Mg25/CoFeB20/Mg50/CoFeB20/TaN20/Mg7/NiCr50 configuration. Each seedlayer stack was evaluated by using a TEM to determine a peak to peakroughness of the uppermost NiCr layer top surface. We found the peak topeak roughness of 2 nm for the TaN/Mg/NiCr stack was significantlydecreased to only 0.5 nm for the seed layer stack with the laminatedsmoothing layer. Therefore, a smoother template layer top surface isachieved by inserting a smoothing layer in the seed layer stack and isbelieved to be responsible for the advantage of substantiallymaintaining PMA in an overlying magnetic layer after high temperatureprocessing such as annealing to about 400° C. for an extended period oftime, typically 1 minute to 10 hours.

The seed layer stack of the embodiments described herein is formed byemploying conventional processes and materials without any significantadded cost and can readily be implemented in a manufacturingenvironment.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

We claim:
 1. A multilayer structure for reducing film roughness in amagnetic device, comprising: (a) a first layer made of a material with afirst bond energy, and having a first surface with an “as deposited”first peak to peak roughness; and (b) an upper second layer that isnon-crystalline or nano-crystalline and is made of a material with asecond bond energy that is greater than the first bond energy such thatdeposition of the upper second layer results in resputtering of thefirst layer to give a first layer with a second surface having a secondpeak to peak roughness substantially less than the “as deposited” firstpeak to peak roughness, and the upper second layer formed on the secondsurface, the upper second layer has a third surface with the second peakto peak roughness.
 2. The multilayer structure of claim 1 furthercomprised of a template layer formed on the third surface of the uppersecond layer and with a top surface having the second peak to peakroughness, the template layer has a (111) crystal orientation to promoteperpendicular magnetic anisotropy (PMA) in an overlying magnetic layer.3. The multilayer structure of claim 1 further comprising a buffer layerthat is Ta or TaN and is formed on a substrate, the buffer layercontacts a bottom surface of the first layer.
 4. The multilayerstructure of claim 2 wherein the template layer is one of NiCr andNiFeCr.
 5. The multilayer structure of claim 1 wherein the first layeris one or more of Mg, Al, Si, C, B, Mn, Rb, Zn, and Ti.
 6. Themultilayer structure of claim 1 wherein the upper second layer is one ofTaN, SiN, and a CoFeM alloy wherein M is one of B, P, Ta, Zr, Si, Cu,Hf, Mo, W, and Nb with a content which makes the CoFeM alloy amorphousas deposited.
 7. The multilayer structure of claim 1 wherein the firstlayer has a thickness from about 3 to 100 Angstroms.
 8. The multilayerstructure of claim 1 wherein the upper second layer has a thickness fromabout 1 to 100 Angstroms.
 9. The multilayer structure of claim 2 whereinthe overlying magnetic layer contacts the top surface of the templatelayer, and is a reference layer in a magnetic tunnel junction (MTJ)having a bottom spin valve configuration, or is a free layer in a MTJwith a top spin valve configuration.
 10. The multilayer structure ofclaim 1 wherein the overlying magnetic layer is a reference layer, freelayer, or dipole layer in a magnetic random access memory (MRAM) device,spin torque oscillator (STO), spintronic device, or a read head sensor.11. A magnetic tunnel junction (MTJ), comprising: (a) a first seed layer(SL1) stack, comprising: (1) a smoothing layer formed on a substrate,the smoothing layer comprises a first layer with a first resputteringrate, and an upper second layer that is non-crystalline ornano-crystalline and has a second resputtering rate wherein the firstresputtering rate is about 2 to 30 times greater than the secondresputtering rate; and (2) a template layer formed on a top surface ofthe upper second layer and with a top surface having a peak to peakroughness of about 0.5 nm, the template layer has a (111) crystalorientation to promote PMA in an overlying magnetic layer; (b) areference layer (RL) formed on the template layer; (c) a free layer(FL); and (d) a tunnel barrier formed between the reference layer andfree layer to provide a SL1/RL/tunnel barrier/FL configuration.
 12. TheMTJ of claim 11 further comprising a capping layer to give aSL1/RL/tunnel barrier/FL/capping layer configuration.
 13. The MTJ ofclaim 11 further comprising a buffer layer that is one of Ta or TaNformed on the substrate and contacting a bottom surface of the smoothinglayer.
 14. The MTJ of claim 11 wherein the template layer is one of NiCrand NiFeCr.
 15. The MTJ of claim 10 wherein the first layer is one ormore of Mg, Al, Si, C, B, Mn, Rb, Zn, and Ti.
 16. The MTJ of claim 11wherein the upper second layer is one of TaN, SiN, and a CoFeM alloywherein M is one of B, P, Ta, Zr, Si, Cu, Hf, Mo, W, and Nb with acontent which makes the CoFeM alloy amorphous as deposited.
 17. The MTJof claim 11 wherein the first layer has a thickness from about 3 to 100Angstroms.
 18. The MTJ of claim 11 wherein the upper second layer has athickness from about 1 to 100 Angstroms.
 19. A magnetic tunnel junction(MTJ), comprising: (a) a first seed layer (SL1) stack, comprising: (1) asmoothing layer formed on a substrate, the smoothing layer comprises afirst layer with a first resputtering rate, and an upper second layerthat is non-crystalline or nano-crystalline and has a secondresputtering rate wherein the first resputtering rate is about 2 to 30times greater than the second resputtering rate; and (2) a templatelayer formed on a top surface of the upper second layer and with a topsurface having a peak to peak roughness of about 0.5 nm, the templatelayer has a (111) crystal orientation to promote PMA in an overlyingmagnetic layer; (b) a free layer (FL) formed on the template layer; (c)a tunnel barrier layer on the free layer; (d) a reference layer (RL) onthe tunnel barrier layer; and (e) a capping layer to provide aSL1/FL/tunnel barrier/RL/capping layer configuration.